This invention relates generally to a heat and pressure fuser for an electrophotographic printing machine, and more particularly the invention is directed to a thermistor for measuring the surface temperature of a heated fuser member.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip.
Thermistors are typically used to sense the temperature of a fuser roll in xerographic machines. Every fusing surface mounted temperature sensor requires a wear tape. This tape minimizes the the effects of wear on the fuser roll due to the intimate contact of the temperature sensor. The tape however does eventually wear. If the tape is not replaced prior to the tape completely wearing through, fuser roll damage will occur. Historically the entire thermistor assembly is replaced. This assembly normally consists of a plastic housing, foam, wires, electrical connector, plus the thermistor bead. A typical 1090 style temperature assembly cost $6.11 (1994 dollars). For 1090 family, 4850, & 4890 machines the replacement rate for these assemblies is one or two for every 1.2-1.5 per million prints. The 4890 machine incorporates two sensors in its design, thus doubling the cost of discarding the thermistor assembly once the tape has worn.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 5,281,793 granted to Gavin et on Jan. 25, 1994 discloses an apparatus for positioning a temperature sensing element in temperature sensing relationship with a moving object. The apparatus includes a base and also a resilient member, attached to the base, for supporting the sensing element. The apparatus further includes a mechanism for urging the sensing element toward the moving object. Moreover, the apparatus includes a mechanism, interposed between the resilient member and the moving object, for reducing frictional resistance between the apparatus and the moving object. The apparatus additionally includes a mechanism, removably attachable to the base, for securing the resistance reducing mechanism at a position interposed between the resilient member and the moving object. In this patent, a portion of a tape is interposed between a resilient member and a fuser roll for minimizing friction therebetween. The tape is fabricated from a polyimide film. By way of example, tape can be made from KAPTON, a trademark of E. I. duPont de Nemours & Co., Inc. of Wilmington, Del.
U.S. Pat. No. 5,194,890 granted to Haruna et al on Mar. 16, 1993 discloses a temperature sensor is fixed on the copying apparatus so that it makes contact with the surface of the heating roller which is coated with teflon (tetrafluoroethylene). The temperature sensor is covered with kapton resin to improve the wear-resistance.
U.S. Pat. No. 3,914,862 granted to McBride et al discloses a method of making a sensor wherein a cylindrical Kapton sleeve used for insulating the side walls of a can.
Japanese publication 59-44633 dated Mar. 13, 1984 discloses a temperature detector for a fixing roll wherein a polyimide sheet is made to adhere as one body to a plastic substrate.
U.S. Pat. No. 4,821,062 granted to Katoh et al on Apr. 11, 1989 discloses a thermistor employing a tape of heat resistive material such as Teflon of polyimide resin. The tape is adhered to the thermistor assembly at a position between a heat roller and the thermistor to prevent the roller from being damaged.
As will be appreciated, it would be highly desirable to be able to easily replace a worn strap or tape that costs 37 cents rather than an entire thermistor assembly.